It is well recognized that primary hydrocarbon recovery techniques may recover only a portion of the petroleum in the formation. Thus, numerous secondary and tertiary recovery techniques have been suggested and employed to increase the recovery of hydrocarbons from the formations holding them in place. Thermal recovery techniques have proven to be effective in increasing the amount of oil recovered from the ground. Water flooding and steam flooding have proven to be the most successful oil recovery techniques yet employed in commercial practice, however, the use of these techniques may still leave up to 60% to 70% of the original hydrocarbons in place, depending on the formation and the quality of the oil.
Furthermore, steam flooding can be a very expensive proposition. The oil remaining in a formation may not be worth the high cost of steam injection and production. This is particularly true for light gravity oil reservoirs, especially those which have been previously subjected to water flooding.
The problem in successfully applying steam flooding to light gravity oil reservoirs is associated with process economics and more particularly with incremental oil saturation. In a traditional steam flood application for a heavy oil holding, a change in oil saturation of up to 0.5 and 0.6 are representative oil recovery targets. In conventional and light oil holdings, where water flooding techniques have already been applied, the initial oil saturation value before steam flooding may be as low as 0.3 to 0.4. Even when such as initial oil saturation is reduced to a possible 5% residual oil saturation, a reduction in oil saturation of only 0.25 to 0.35 is obtained. Generally, an incremental recovery goal of 0.25 to 0.35 will not economically support full scale steam flood efforts. Consequently, investigations have been conducted into possible modifications of steam flooding.
It is old in the art to use lower quality steam in a continuous injection manner. A second method is disclosed in U.S. Pat. No. 3,360,045 wherein steam injection is followed by hot water containing a polymer to increase viscosity. A third process is disclosed in U.S. patent application Ser. No. 392,415, filed June 25, 1982, to a varying temperature oil recovery method for heavy oils. In this process, initial injection is begun with ambient temperature water, followed by water of a gradually increasing temperature until 100.degree. C. is reached, followed by steam of a low quality wherein the steam quality gradually increases, followed by a steam flood with high quality steam.
U.S. patent application Ser. No. 463,214, filed concurrently herewith on Feb. 2, 1983, discloses a fourth method for reducing the total quantity of steam injected. This method advocates the use of a small steam slug sufficient to generate a steam distillation front, followed by a slug of non-condensable gas to prevent steam front collapse upon injection of cold water. All of these processes reduce the cost of a usual steam flood and attempt to get oil recoveries similar to that of full-scale steam floods.